As a light source of an image display apparatus such as a projector, a lamp unit including a concave reflecting mirror into which a high pressure discharge lamp is built is commonly used. As a technology to provide improved light to a screen in such a lamp unit, Patent Literature 1 discloses an arc tube having a shape that enables light radiated from a high pressure discharge lamp to be captured by a reflecting mirror efficiently. According to the structure of the arc tube, a radius of curvature of an outer surface of a light-emitting part is smaller in portions close to sealing parts than in a central portion. Therefore, light passing though the portions close to the sealing parts is radiated from the light-emitting part in a direction substantially perpendicular to a tube axis of the arc tube. The high pressure discharge lamp including the arc tube exhibits favorable light intensity distribution properties due to a high level of light radiated in a direction perpendicular to the tube axis of the arc tube, and therefore, is capable of focusing a large amount of light on a reflecting surface of the reflecting mirror. Therefore, it becomes possible to provide improved light to a screen in a lamp unit.